A UMTS communication system, defined in the UTMTS standard (see the “3GPP Technical Specifications, Rel. 99”), at the 3GPP website under specifications, incorporated herein by reference, comprises at least a base station (BS) and a mobile phone terminal, referred to as user equipment (UE) throughout the document. Both the BS and the UE includes a transmitter and a receiver. The BS sends signals by means of the BS transmitter to the UE receiver in the downlink. The UE sends signals by means of the UE transmitter to the BS receiver in the uplink. In uplink and downlink communication both the UE and BS map the data to be transmitted into logical channels.
In particular the UMTS standard provides two logical channels for the downlink communication, a first channel denoted common pilot channel (CPICH) consisting of a continuous sequence of known pilot symbols and a second logical channel denoted dedicated physical channel (DPCH). Said second logical channel DPCH consists of a first logical sub-channel denoted dedicated physical data channel (DPDCH), that carries the information data symbols intended for the user concerned, time multiplexed with a second logical sub-channel denoted dedicatedphysical control channel (DPCCH), that carries known user-dedicated pilot symbols.
Transmit beamforming can be employed at the BS during downlink communication to improve the reception at the UE of the user information data mapped into the logical channel DPDCH. To perform transmit beamforming, the BS needs to be equipped with multiple antennas. In this way it is possible to provide optimal spatial filtering by shaping beams in the directions of the user concerned while setting nulls in other users directions to mitigate the interference to other users. Hence the use of transmit beamforming can significantly improve the efficiency in the use of the spectrum, allowing a larger number of users to share the same spectrum, thanks to an improved user-dedicated data reception and reduced interference. While transmit beamforming is to be employed for the transmission of the logical channel DPCH, it is not suited for the logical channel CPICH transmission that is to be uniformly broadcast in time and space (i.e. in all directions) to all users. As seen from the UE receiver, the transmission of the logical channel DPCH occurs through a so called DPCH propagation channel consisting of the cascade of the BS transmitter chain, including the beamforming antenna system, the actual propagation channel from the BS to the UE and the UE receiver chain. Unlike the logical channel CPICH, downlink transmission occurs through a so-called CPICH propagation channel consisting of the cascade of the BS transmitter chain, including an omnidirectional antenna system in place of the beamforming antenna system, of the actual propagation channel from the BS to the UE and the receiver chain. For the sake of brevity we shall refer to a DPCH channel and a CPICH channel and the above DPCH propagation channel and CPICH propagation channel respectively throughout the present document. The UE receiver needs to estimate the DPCH channel in order to demodulate, decode, and recover the information data contained in the DPDCH. In the absence of transmit beamforming the CPICH channel and the DPCH channel are the same (up to an uninfluential real scalar factor due to the different transmit power allocated to the DPDCH, DPCCH and CPICH logical channels). The continuous provision of known pilot symbols of the CPICH logical channel can be easily used at the UE receiver to estimate the CPICH channel, which is the same as the DPCH channel, and then use this estimate for DPDCH information data recovery. Unfortunately, when transmit beamforming is applied, the known pilots of the CPICH logical channel yet provide estimates for the CPICH channel, but those estimates cannot be directly applied as channel estimates for the DPCH channel that is different from the CPICH channel. Then the UE receiver has to rely on different techniques to estimate the DPDCH channel in order to recover the information data.